1. Field of the Invention
This invention relates generally to the field of charged particle optical columns, and more particularly to methods and apparatus for charged particle beam blanking.
2. Description of the Related Art
The use of electron beams is an established technique used to write on a resist on the surface of a substrate to be patterned. Applications of electron-beam lithography include writing of masks and reticles for use in semiconductor photolithography, and electron-beam direct-write (EBDW) on semiconductor wafers. In these applications, in order to write a pattern, it is necessary to be able to turn the beam on and off at the substrate in a rapid and controlled manner. To do this, a device called a “beam blanker” is commonly employed, as is familiar to those skilled in the art. A beam blanker is typically a pair of flat electrodes, one located on each side of the beam in an electron optical column. FIGS. 1 and 2 illustrate two embodiments of a prior art beam blanker.
Beam Blanking by a Single Pair of Blanker Plates
FIG. 1 is a schematic side cross-sectional view of a first prior art beam blanker deflecting a charged particle beam. A converging electron beam 702 moves downwards (arrow 703) towards a pair of blanker plates 706 and 708, located symmetrically on each side of optical axis 704. Blanker plates 706 and 708 have flat inner surfaces extending above and below the plane of the figure. Voltage supply V1 736 is connected to plate 706, while voltage supply V2 738 is connected to plate 708. In FIG. 1, V1<0 and V2=−V1>0, thus a horizontal electric field is induced between plates 706 and 708 which deflects the electron beam 702 to the right as shown. Ignoring end effects at the tops and bottoms of plates 706 and 708, the beam is deflected smoothly as shown in FIG. 1, passing through a crossover 716, then diverging as beam 712 passes out from the space between blanker plates 706 and 708. Deflection angle 724 shows the angle between the incoming beam 702 and the outgoing beam 712, which is centered on axis 714 with direction 713. Although the actual beam deflection due to the pair of blanker plates 706 and 708 is a smooth curve, the overall beam deflection 724 can be approximated by an step-function change in beam direction centered in plane 710 (the mid-plane of plates 706 and 708), with the same deflection angle 724 as is shown for the actual beam trajectory through the blanker. Plane 710 is commonly called the “effective blanking plane”. In this approximation, beam 702 is extrapolated between the tops of plates 706 and 708 down to plane 710 by virtual trajectories 718, which converge to a virtual crossover 720. Below the virtual crossover 720, virtual trajectories 722 extend to the bottom of plates 706 and 708, asymptotically converging to the actual beam profile 712. This approximation is used throughout FIGS. 3-10A, 11, 12A, 13, and 14A for simplicity of illustration of both the prior art and the present invention.
Beam Blanking by a Double Pair of Blanker Plates
FIG. 2 is a schematic side cross-sectional view of a second prior art beam blanker deflecting a charged particle beam, comprising two pairs of blanker electrodes. One pair (electrodes 806 and 808) is positioned above the effective blanking plane 810 and the other pair (electrodes 826 and 828) is positioned below the effective blanking plane 810. With this alternative configuration, there is the capability to position a blanking aperture in the effective blanking plane, shown as two plates 841 and 842. Alternatively, the blanking aperture may be a single plate with a hole on optical axis 804.
A converging electron beam 802 moves downwards (arrow 803) into the gap between a first pair of blanker plates 806 and 808, located symmetrically on each side of optical axis 804. Blanker plates 806 and 808 have flat inner surfaces extending above and below the plane of the figure. A second pair of blanker plates 826 and 828 is positioned below the first pair as shown. The effective blanking plane 810 is located between the first pair and the second pair of blanker electrodes. Upper plates 806 and 808 deflect the beam off-axis and onto either plate 841 or plate 842, thereby blanking the beam (FIG. 2 shows beam 802 with almost enough deflection to hit plate 842). Lower plates 826 and 828 enable conjugate blanking by maintaining the virtual crossover 820 on-axis, even while the actual crossover 816 is moving off-axis due to the beam deflection induced by plates 806 and 808. Voltage supply V1 836 is connected to plates 806 and 826, while voltage supply V2 838 is connected to plates 808 and 828. In FIG. 2, V1<0 and V2=−V1>0, thus two horizontal electric fields are induced: a first field between plates 806 and 808, and a second field between plates 826 and 828. The combined effect of the two electric fields is equivalent to the effect of the electric field between plates 706 and 708 in FIG. 1. The combined influence of the two electric fields deflects the electron beam 802 to the right as shown. Ignoring end effects at the tops and bottoms of plates 806, 808, 826, and 828, the beam is deflected smoothly as shown in FIG. 2, passing to a crossover 816, then diverging as beam 812 passes out from the space between blanker plates 826 and 828. Deflection angle 824 shows the angle between the incoming beam 802 and the outgoing beam 812, which is centered on axis 814 with direction 813. Although the actual beam deflection due to the two pairs of blanker plates is a smooth curve, the overall beam deflection 824 can be approximated by an step-function change in beam direction centered in plane 810, as for plane 710 in FIG. 1. Because plates 806 and 828 are connected to the same voltage supply 836, and plates 808 and 828 are connected to the same voltage supply 838, the position of the effective blanking plane is fixed in this prior art embodiment.
The Need for Conjugate Blanking in an Electron-Beam Lithography System
FIG. 3 is a schematic side cross-sectional view of a prior art electron beam column showing an unblanked beam with a crossover at the center of a single pair of blanker plates. Electrons are shown being emitted from a source tip 102 at the top of the column to form a diverging beam 104. The electron source can be a cold field emitter, a Schottky thermal field emitter, a LaB6 thermal emitter, a thermionic source, or any other type of electron emitter—the particular type of electron source is not part of the present invention. Beam 104 is focused into a converging beam 108 by lens 106, commonly called a “gun lens”. Source 102, lens 106, blanking aperture 120, and lens 124 are all centered on symmetry axis 103. Lens 106 forms a crossover 114 on axis 103 at the effective blanking plane 116 of the blanker comprising plates 110 and 112, which extend above and below the planes of FIGS. 3-8. Voltage supply V1 130 is connected to plate 110, while voltage supply V2 132 is connected to plate 112. In FIG. 3, V1=V2=0 (i.e., the blanker is not activated), thus there is no electric field induced between plates 110 and 112 and beam 108 above crossover 114, and beam 118 below crossover 114, are not deflected off axis 103. Because the diverging beam 118 is undeflected by the blanker, a portion of the electrons in beam 118 passes through aperture 120, to form diverging beam 122. Beam 122 is then focused into a converging beam 126 by lens 124, commonly called the “objective lens”, or “main lens”. Beam 126 is focused onto the surface of substrate 128 at image 129 by lens 124. The beam at image 129 is a focused image of the virtual object at the beam crossover 114. The overall magnification of the electron source 102 at the image 129 is determined by the position of crossover 114 in relation to source 102, lenses 106 and 124, and the substrate 128, as is familiar to those skilled in the art.
FIG. 4 is a schematic side cross-sectional view of a prior art electron beam column showing a partially blanked beam with a crossover at the center of a single pair of blanker plates. As in FIG. 3, electrons from source 102 form a diverging beam 104 which is focused into a converging beam 108 by lens 106. Lens 106 forms a crossover 154 on axis 103 at the mid-plane 116 of the pair of blanker plates 110 and 112. In FIG. 4, V1<0 and V2=−V1>0, thus a horizontal electric field is induced between plates 110 and 112 which deflects the electron beam 158 to the right as shown. Note that the approximation of a step-function beam deflection illustrated in FIG. 1 is used here—the curvature of the actual electron trajectories while passing between plates 110 and 112 is not shown. Beam 158 represents a partially-blanked beam in the column since fewer electrons in beam 158 pass through aperture 120 than in beam 118 in FIG. 3. Those electrons that do pass through aperture 120 form diverging beam 162, which is focused into converging beam 166 by lens 124. Beam 166 is focused onto the surface of substrate 128 at image 169 by lens 124. The beam at image 169 is a focused image of the virtual object at the beam crossover 154. Because the blanker effectively “pivots” the beam at the crossover 154, image 169 is on axis 103—this is called “conjugate blanking”, as is familiar to those skilled in the art. The importance of conjugate blanking for an electron beam lithography system is that as the beam is blanked on and off, the location of the focused image on the substrate surface (such as image 129 in FIG. 3, and image 169 in FIG. 4) does not move. If the focused image were to move during beam blanking or unblanking, then the pattern written on the substrate would be blurred, which is clearly undesirable for precision patterning. Thus, FIGS. 3 and 4 illustrate the ideal case where the beam crossover is at the effective blanking plane and thus conjugate blanking is ensured.
In order to ensure conjugate blanking in the column illustrated in FIGS. 3 and 4, it is clearly necessary to maintain the beam crossovers 114 and 154 at the effective blanking plane 116—this means that the overall magnification from the source 102 to the images 129 and 169 must remain fixed, since the only means for changing the magnification is by moving the position of the crossover. For example, to reduce the magnification, the crossover would be moved up, as in FIGS. 5 and 6, while to increase the magnification, the crossover would be moved down, as in FIGS. 7 and 8. The problem with the prior art blanker can now be seen—if it is necessary to change the column magnification, a simple two-plate blanker as shown in FIGS. 3-8 cannot preserve conjugate blanking. This is discussed in more detail for FIGS. 5-8.
Non-Conjugate Blanking—Crossover Above the Effective Blanking Plane
FIG. 5 is a schematic side cross-sectional view of a prior art electron beam column showing an unblanked beam with a crossover above the center of a single pair of blanker plates, corresponding to a situation in which the magnification of the source at the substrate is smaller than in FIGS. 3 and 4. Electrons are shown being emitted from a source tip 102 at the top of the column to form a diverging beam 204. Beam 204 is focused into a converging beam 208 by lens 206. Lens 206 is physically equivalent to lens 106 in FIGS. 3 and 4, however the focusing strength of lens 206 has been increased relative to lens 106 to move crossover 214 higher in the column (along axis 103) than crossovers 114 and 154 in FIGS. 3 and 4, respectively. Since the position of the effective blanking plane 116 is unchanged, crossover 214 (in plane 117) is now no longer at the position required to produce conjugate blanking. In FIG. 5, V1=V2=0 (i.e., the blanker is not activated), thus there is no electric field induced between plates 110 and 112 and beam 208 above crossover 214, and beam 218 below crossover 214, are not deflected off axis 103. Because the diverging beam 218 is undeflected by the blanker, a portion of the electrons in beam 218 passes through aperture 120, to form diverging beam 222. Beam 222 is then focused into a converging beam 226 by lens 224. Lens 224 is physically equivalent to lens 124 in FIGS. 3 and 4, however the focusing strength of lens 224 has been decreased relative to lens 124 to compensate for the higher position of crossover 214, which is the virtual object for lens 224. Control of the focusing strengths of lenses 206 and 224 may be accomplished by changing the excitation current (for magnetic lenses), or by changing the voltages of one or more electrodes (for electrostatic lenses). Beam 226 is focused onto the surface of substrate 128 at image 229 by lens 224. Because in FIG. 5 the blanker comprising plates 110 and 112 is not activated, image 229 still falls on axis 103.
FIG. 6 is a schematic side cross-sectional view of a prior art electron beam column showing a partially blanked beam with a crossover above the center of a single pair of blanker plates. As in FIG. 5, electrons from source 102 form a diverging beam 204 which is focused into a converging beam 208 by lens 206. Lens 206 forms a crossover 254 in plane 117, which is above the effective blanking plane 116, as in FIG. 5. Beam 257 below crossover 254 is a diverging beam which can be treated as having a step-function deflection at the effective blanking plane 116 as shown in FIG. 1. In FIG. 6, V1<0 and V2=−V1>0, thus a horizontal electric field is induced between plates 110 and 112 which deflects the electron beam 258 to the right as shown. Virtual rays 256 are extrapolated upwards (above the effective blanking plane 116) from rays 258—the apparent source of rays 256 determines the position of the virtual object 255. Beam 258 represents a partially-blanked beam in the column since fewer electrons pass through aperture 120 in beam 258 than in beam 218 in FIG. 5. Those electrons that do pass through aperture 120 form diverging beam 262, which is focused into converging beam 266 by lens 224. Beam 266 is focused onto the surface of substrate 128 at image 269 by lens 224. The beam at image 269 is a focused image of the virtual object 255. Because crossover 254 is not in the effective blanking plane 116, the virtual object 255 for lens 224 appears to be offset to the left, as shown. For lens 224, the offset of virtual object 255 means that the image 269 on substrate 128 is offset to the right, as is familiar to those skilled in the art. Because image 269 is no longer on axis 103, there is non-conjugate blanking and the pattern written on substrate 128 will be blurred.
Non-Conjugate Blanking—Crossover Below the Effective Blanking Plane
FIGS. 7 and 8 illustrate the opposite case from FIGS. 5 and 6: the crossover is now below the effective blanking plane 116, corresponding to a situation in which the magnification of the source at the substrate is larger than in FIGS. 3 and 4.
FIG. 7 is a schematic side cross-sectional view of a prior art electron beam column showing an unblanked beam with a crossover 314 below the center of a single pair of blanker plates 110 and 112. Electrons are shown being emitted from a source tip 102 at the top of the column to form a diverging beam 304. Beam 304 is focused into a converging beam 308 by lens 306. Lens 306 is physically equivalent to lens 106 in FIGS. 3 and 4, however the focusing strength of lens 306 has been decreased relative to lens 106 to move crossover 314 lower than crossovers 114 and 154 in FIGS. 3 and 4, respectively. Since the position of the effective blanking plane 116 is unchanged, crossover 314 (in plane 317) is now no longer at the position required to produce conjugate blanking. In FIG. 7, V1=V2=0 (i.e., the blanker is not activated), thus there is no electric field induced between plates 110 and 112 and beam 308 above crossover 314, and beam 318 below crossover 314, are not deflected off axis 103. Because the diverging beam 318 is undeflected by the blanker, a portion of the electrons in beam 318 passes through aperture 120, to form diverging beam 322. Beam 322 is then focused into a converging beam 326 by lens 324. Lens 324 is physically equivalent to lens 124 in FIGS. 3 and 4, however the focusing strength of lens 324 has been increased relative to lens 124 to compensate for the lower position of crossover 314, which is the virtual object for lens 324. Beam 326 is focused onto the surface of substrate 128 at image 329 by lens 324. Because in FIG. 7 the blanker comprising plates 110 and 112 is not activated, image 329 still falls on axis 103.
FIG. 8 is a schematic side cross-sectional view of a prior art electron beam column showing a partially blanked beam with a crossover below the center of a single pair of blanker plates. As in FIG. 7, Electrons from source 102 form a diverging beam 304 which is focused into a converging beam 308 by lens 306. Lens 306 forms a crossover 354 in plane 117, which is below the effective blanking plane 116, as in FIG. 7. In FIG. 8, V1<0 and V2=−V1>0, thus a horizontal electric field is induced between plates 110 and 112 which deflects the electron beam 357 to the right as shown. Beam 308 above the effective blanking plane 116 is a converging beam which can be treated as having a step-function deflection at the effective blanking plane 116 as shown in FIG. 1. Note that in this example, the beam crossover 354 is off-axis to the right in plane 317, in contrast with the situation for the virtual crossover 255 in FIG. 6. Beam 358 represents a partially-blanked beam in the column since fewer electrons in beam 358 pass through aperture 120 than in beam 318 in FIG. 7. Those electrons that do pass through aperture 120 form diverging beam 362, which is focused into converging beam 366 by lens 324. Beam 366 is focused onto the surface of substrate 128 at image 369 by lens 324. The beam at image 369 is a focused image of the beam crossover 354. For lens 324, the offset to the right of crossover 354 means that the image 369 on substrate 128 is offset to the left, as is familiar to those skilled in the art. Because image 369 is no longer on axis 103, there is non-conjugate blanking and the pattern written on substrate 128 will be blurred, as in FIG. 6.
In conclusion, there is a need for a method and apparatus for conjugate blanking of charged particle beams, which accommodates charged particle systems where there is no beam cross-over or the cross-over moves axially due to changes in lens excitations.